The present invention relates generally to an implantable device for promoting the fusion of adjacent bony structures, and a method of using the same. More specifically, the present invention relates to an expandable fusion cage that may be inserted into an intervertebral space, and a method of using the same.
Fusion cages provide a space for inserting a bone graft between adjacent portions of bone. Such cages are often made of titanium and are hollow, threaded, and porous in order to allow a bone graft contained within the interior of the cage of grow through the cage into adjacent vertebral bodies. Such cages are used to treat a variety of spinal disorders, including degenerative disc diseases such as Grade I or II spondylolistheses of the lumbar spine.
The majority of spinal fusion cages are placed in front of the spine, a procedure known as anterior lumbar interbody fusion, or ALIF. The cages are generally inserted through a traditional open operation, though laparoscopic or percutaneous insertion techniques may also be used. Cages may also be placed through a posterior lumbar interbody fusion, or PLIF, technique, involving placement of the cage through a midline incision in the back.
Regardless of the approach, the typical procedure for inserting a common threaded and impacted fusion cage is the same. First, the disc space between two vertebrae of the lumbar spine is opened using a wedge or other device on a first side of the vertebrae. The disk space is then prepared to receive a fusion cage. Conventionally, a threaded cage is inserted into the bore and the wedge is removed. A disk space at the first side of the vertebrae is then prepared, and a second threaded fusion cage inserted into the bore. Alternatively, the disk space between adjacent vertebrae may simply be cleared and a cage inserted therein. Often, only one cage is inserted obliquely into the disk space. Use of a threaded cage may be foregone in favor of a rectangular or pellet-shaped cage that is simply inserted into the disk space.
Although ALIF is common, the procedure suffers from disadvantages. In cases where patients have a “tall” disc, or where there is instability (such as with isthmic spondylolistheses), an anterior approach to the spinal fusion may not provide adequate stability. Further, the procedure is performed in close proximity to the large blood vessels that go to the legs, thereby risking damage to these blood vessels, which can result in excessive blood loss. In dealing with male patients, another unique risk arises. Approaching the L5-S1 disc space from the front risks a condition known as retrograde ejaculation. This is due to the position of small nerves directly over the disc interspace that control a valve causing the ejaculate to be expelled during intercourse. Dissecting over the disk space can cause the nerves to stop working and, absent innervation to the valve, the ejaculate may move into the bladder.
A problem common to many fusion cages, regardless of method of insertion, concerns maintaining or restoring the normal anatomy of the fused spinal segment. Once a disc or a portion thereof is removed, the normal lordotic or kyphotic curvature of the spine is eliminated. Traditional fusion cages neglect the need to correct this curvature. Such cages may lead to a kyphotic deformity as the vertebrae settles around the implant. Often, revision surgeries are necessary to correct spinal imbalances. Fusion cages have been designed having a wedge-like shape in order to address these issues, but because of the shape of the cage, such devices must heretofore have been implanted using an ALIF procedure, thereby suffering from all of the disadvantages of using that procedure.
A problem with existing titanium cages is that it is difficult to assess spinal fusions postoperatively because the metal of the cage interferes with attempts to evaluate the fusion by x-ray. Radiolucent cages, such as those made from either carbon fiber or polyetheretherketone (PEEK), have been used to provide better postoperative visualization of spinal fusions. A problem with such cages, however, is that they do not adhere well to the bony endplates and thus often must be supplemented with pedicle screws.
What is needed, therefore, is a spinal fusion cage suitable for a PLIF procedure that allows for preservation or restoration of the proper lordotic or kyphotic curvature of the spine, provides adequate strength and stability to be used with or without supplements such as pedicle screws, and that can be visualized postoperatively via radiologic procedures such as x-rays and the like. What is further needed is a fusion cage adapted to remain within an intervertebral space, designed to match the natural curvature of the adjacent vertebrae, and an insertion tool adapted to easily insert such a fusion cage into an intervertebral space. Furthermore, a method is needed for producing a fusion cage having the above characteristics, as well as for producing other implantable devices conforming to the shape of the space to be occupied by the device.